1. Field of the Invention
The present invention relates to a radiation source, for example for producing extreme ultraviolet radiation (EUV) or soft X-ray radiation, which may be used in lithography.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to, generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers,in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Radiation sources are required for generating the radiation to expose the pattern onto the substrate. With the desire to image ever-smaller features, there has been a drive towards using shorter wavelength radiation, and suitable sources,required.
Gas-discharge radiation sources are known for emitting EUV when an electrical discharge is made through vaporized metal between electrodes. The discharge area needs to have a low pressure in order to achieve good conversion efficiency of electrical energy to EUV radiation. A first problem is that a gas buffer is typically provided near the discharge area to stop debris from the discharge reaching other components. The buffer gas flushed towards the discharge area makes it difficult to maintain a low pressure in the vicinity of the discharge. Another problem is that the discharge area needs to be shielded from the environment and previously a metal plate surrounding the discharge area has been provided in order to minimize the inductance of the source environment. The metal plate effectively closes the source chamber, except where the radiation is to be emitted. This also makes pumping to reduce the pressure of the source environment very ineffective. With a prior apparatus, the conversion efficiency can be reduced by a factor of two because of the problem with pumping to maintain a low pressure in the vicinity of the discharge area. A yet further problem with the prior art is adequately cooling the electrodes of the discharge apparatus. This can result in further problems,such as increased electrical resistance which increases the electrical time constant of the circuit and so impairs the performance of the radiation source.